The present invention relates to a method of recycling a lead-acid storage battery whose capacity has been reduced due to a film of lead sulfate formed on a surface of an electrode.
As one type of a secondary battery which can be repeatedly charged and discharged, a lead-acid storage battery is often used for electric motor vehicles and other purposes since it is relatively cheap. Some examples of the lead-acid storage battery are defined in JIS C8701 (portable storage battery), JIS D5301 (storage battery for automobiles), JIS D 5302 (small-sized storage battery for automobiles), JIS D 5303 (storage battery for electric vehicles), JIS F 8101 (storage battery for boats), JIS W 7301 (storage battery for airplanes). However, when the time of use or service of such a storage battery amounts to, for example, about 2-3 years, there generally occurs a phenomenon that the charge capacity is considerably reduced due to the chronological deterioration. In the occurrence of such a phenomenon called xe2x80x9cbattery exhaustionxe2x80x9d, it is generally determined that the service life is over when the actual charge capacity is reduced to, for example, about 50% of the initial charge capacity, so that the battery is replaced with a new one.
A main cause of the above phenomenon, i.e., about 70-80% of the cause of the phenomenon, is development of so-called xe2x80x9csulfationxe2x80x9d in which a hard film consisting of large crystals of lead sulfate (PbSO4) is formed on the surfaces of the electrode plates after the battery is left discharged. It is common that the electrode plate consists of, for example, a sponge structure having a surface in which a multiplicity of small perforations are formed in the interest of raising the capacity of the battery. The small perforations are closed by the solidified non-conducting lead sulfate in the xe2x80x9csulfationxe2x80x9d, thereby reducing the battery capacity while considerably increasing the internal resistance, as a result of reduction of a surface area of the electrode plate which can contribute to charging and discharging of the battery. It is known that the reaction is promoted so that a fatal damage is given to the lead-acid storage battery, particularly, when the load applied to the battery is held constant in spite of the reduction of the surface area of dischargeable electrode plate due to the deposition of the lead sulfate.
Since the lead-acid storage battery whose service life is over as a result of reduction of the capacity contains a large amount of lead, it can not be easily disposed of as an ordinary industrial waste or refuse. Thus, the used lead-acid storage batteries are likely to be piled up out in the open. It would be possible to collect the lead from the used battery in a metal collection treatment. However, such a treatment requires a large investment such as a relatively large plant equipment for a lead refining operation in which crushing/screening, reduction fusion and electrolysis of the lead are effected for refining the lead, leading to an increase in the cost and making it impossible to easily carry it out.
On the other hand, as described in the publication of Japanese Patent No. 2736243 (issued on April 2, Heisei 10), there is proposed a technique of activating the positive electrode (PbO2) of the lead-acid storage battery through an electrochemical doping, by adding a carbon suspension, which is obtained by electrolytic oxidation of a carbon positive electrode in a water system, to a battery electrolyte of the lead-acid storage battery, and also proposed to use the carbon suspension as a regenerant for regenerating the lead-acid storage battery. However, although the activation of the positive electrode is surely achieved by supplementing the battery electrolyte of the used lead-acid storage battery with the carbon suspension, the activation is achieved only in the remaining area of the electrode plate which is not covered by the deposition of the lead sulfate. Thus, in the disclosed technique in which the lead sulfate itself is not removed, it is necessary to select, as recyclable batteries, only those of the used batteries which have a specific gravity of not smaller than about 1.24 and which are accordingly not considered to suffer from the sulfation. Otherwise, it is extremely difficult to restore the capacity of the used battery to the level of capacity of a new battery. That is, the application of the disclosed technique is limited to the recycling of such used lead-acid storage batteries that have relatively better conditions.
The present invention was developed under the above-described background situation. An object to be achieved by the present invention is to provide a method of recycling a wide range of used lead-acid storage batteries.
As a result of various studies made by the present inventors under the above-described background situation, the inventors found out a fact that it is possible to advantageously recycle the used lead-acid storage battery whose capacity has been considerably reduced, by combining the above-described technique of adding the carbon suspension to the battery electrolyte for the purpose of activating the electrode plate, with a technique of charging a lead-acid storage battery of an emergency vehicle which is not usually used but needs to be surely activated in an emergency, by continuously or periodically applying a pulsating direct current having a relatively high frequency of about 10 kHz, to the lead-acid storage battery for the purpose of preventing occurrence of the sulfation. That is, it is considered that the used lead-acid storage battery can be recovered to have substantially the same capacity as that of a new lead-acid storage battery, since an area of the surface of the electrode plate which can contribute to the charging and discharging is restored owing to removal of the lead sulfate deposited on the surface of the electrode plate by charging the battery with the pulsating current having the relatively high frequency, while the electrode plate is activated through the electrochemical doping by adding the carbon suspension to the battery electrolyte. This invention was developed based on this knowledge.
That is, the essence of the present invention is a method of recycling a lead-acid storage battery, to recover a capacity of the battery which has been reduced due to deposition of a lead sulfate on a surface of an electrode of the battery. The method comprises: (a) a first step of causing a pulsating direct current to flow from a positive electrode toward a negative electrode of the battery, for thereby reducing the lead sulfate deposited on the surface of the electrode; and (b) a second step of activating the positive electrode through an electrochemical doping in which a direct current voltage is applied to the battery with a carbon suspension being used as a battery electrolyte of the battery. The carbon suspension is obtained by electrolytic oxidation of a carbon positive electrode in a water system.
In this method, the lead sulfate deposited on the surface of the electrode is reduced owing to the flow of the pulsating direct current from the positive electrode toward the negative electrode in the first step, while at the same time the positive electrode is activated through the electrochemical doping in which the carbon suspension obtained by the electrolytic oxidation of the carbon positive electrode is used as at least a part of the battery electrolyte of the lead-acid storage battery in the second step. Therefore, according to the present invention, even an used lead-acid storage battery which has a battery electrolyte whose specific gravity is considerably lowered and which accordingly appears to suffer from the sulfation can be recovered to have substantially the same capacity as that of a new lead-acid storage battery. Thus, the invention makes it possible to select, as recyclable batteries, a wider range of used lead-acid storage batteries including the ones which are conventionally considered to be unrecyclable, thereby remarkably reducing the used storage batteries to be disposed of as wastes. Further, the adding of the carbon suspension into the battery electrolyte makes it possible to prevent the lead sulfate from adhering to the electrode of the lead-acid storage battery, thereby providing an advantage that it is not necessary to cause periodic or continuous flow of the pulsating direct current.
Preferably, in the first step, the pulsating direct current is caused to flow by applying, to the battery, a pulsating direct current voltage which is sufficiently higher than a terminal voltage of the battery, and wherein the pulsating direct current has a rectangular, sinusoidal or partially arcuate waveform.
Further, preferably, in the first step, the pulsating direct current is caused to flow from the positive electrode toward the negative electrode for a time ranging from 12 to 24 hours, so that the lead sulfate is surely removed from the surface of the electrode.
Further, preferably, the pulsating direct current used in the first step has a frequency ranging from 2 to 12 kHz. If the frequency of the pulsating current is not lower than 12 kHz, the efficiency of removing the lead sulfate is drastically reduced. If the frequency is not higher than 2 kHz, the time required for removing the lead sulfate is increased whereby the working efficiency is reduced.
Further, preferably, the water system used in the second step consists of a pure water system, an aqueous solution of a dilute sulfuric acid, or an aqueous solution including a small amount of additive electrolyte which does not deteriorate performance of the lead-add storage battery when the additive electrolyte is added to a battery electrolyte.
Further, preferably, the carbon suspension used in the second step consists of a colloidal suspension in which surfaces of carbon colloidal particles are chemically modified with a hydraulic group such as a carbonyl group, a carboxyl group and a hydroxyl group.
Further, preferably, the direct current voltage is applied to the battery in the second step such that an electric current flows from the positive electrode toward the negative electrode of the battery, by using a constant current or a pulsating current which changes in magnitude in a predetermined cycle.
Further, preferably, the second step in which the carbon suspension obtained by the electrolytic oxidation of the carbon positive electrode in the water system is added to the battery electrolyte of the battery is implemented before, during or after implementation of the first step in which the pulsating direct current is caused to flow from the positive electrode toward the negative electrode of the battery.